Cancer is characterized by cell proliferation without normal regulation by external signals, and the potential to invade and metastasize to other tissues. For many years chemotherapy has been a mainstay of treatment for various types of cancer. Conventional chemotherapy works essentially by poisoning rapidly dividing cells. As such, it has relatively low selectivity for cancer cells per se, resulting in the familiar side effects of hair loss, diarrhea and other forms of gastrointestinal upset, and marrow suppression. Such off-target side effects frequently become dose-limiting, and typically impose a constraint on treatment efficacy.
For example, doxorubicin, also known as hydroxydaunorubicin, is a drug used in cancer chemotherapy. It is an anthracycline antibiotic, closely related to the natural product daunomycin. Like all anthracyclines, it works by intercalating DNA, with the most serious adverse effect being life-threatening heart damage. Doxorubicin is commonly used in the treatment of a wide range of cancers, including hematological malignancies, many types of carcinoma, and soft tissue sarcomas.
Anticancer therapy would be greatly improved if it were selectively targeted to cancer cells. Many approaches have been proposed and developed with the goal of achieving selective targeting of cancer treatment agents. For example, cytotoxic agents have been linked to monoclonal antibodies and antigen-specific fragments thereof which are capable of binding specifically to certain tumor antigens.
The effect of folate-targeted liposomal doxorubicin (FTL-Dox) has been well characterized in folate receptor (FR)-overexpressing tumors in vitro, particularly in KB human carcinoma cells. Riviere et al. J Drug Targeting 19(1):14-24 (2011) investigated the antitumor activity of FTL-Dox injected intravenously into mice bearing KB tumors. Mice were administered a single intravenous injection of free Dox, nontargeted PEGylated liposomal Dox (PL-Dox), or FTL-Dox. FTLs and PLs accumulated similarly in tumor tissue, despite the faster clearance of FTLs from circulation. Mice treated with FTL-Dox (20 mg/kg) displayed greater inhibition of tumor growth, and almost a 50 percent increase in life span, compared to mice receiving PL-Dox (20 mg/kg). Riviere et al. concluded that while FTLs administered systemically have the potential to enhance the delivery of anticancer drugs in vivo, their removal by FR-expressing normal tissues may have to be blocked if the benefits of tumor targeting are to be realized.
Membrane-bound proteases have recently emerged as critical mediators of tumorigenesis, angiogenesis, and metastasis. Fibroblast activation protein alpha (FAPα, or simply FAP; EC 3.4.21.-), also known as seprase or 170 kDa melanoma membrane-bound gelatinase, is a homodimeric integral membrane protein belonging to the serine protease family. Scanlan et al. (1994) Proc Natl Acad Sci USA 91:5657-61; and WO 97/34927 (incorporated by reference).
Normal adult tissues generally do not express detectable amounts of FAP. In contrast, FAP is expressed in reactive stromal fibroblasts of epithelial cancers, granulation tissue of healing wounds, and malignant cells of bone and soft tissue sarcomas. FAP is thought to be involved in the control of fibroblast growth or epithelial-mesenchymal interactions during development, tissue repair, and epithelial carcinogenesis. Significantly, most common types of epithelial cancers, including more than 90 percent of breast, non-small cell lung, and colorectal carcinomas, contain FAP-expressing stromal fibroblasts. Scanlan et al. Proc Natl Acad Sci USA 91:5657-61 (1994). Because in adults its expression is restricted to pathologic sites, including cancer, fibrosis, arthritis, wounding, and inflammation, FAP can provide target specificity to therapeutic agents.
U.S. Pat. No. 6,613,879 (incorporated by reference) to Firestone et al. discloses a prodrug that is capable of being converted into a cytotoxic or cytostatic drug by catalytic action of human FAP. The prodrug includes a cleavage site which is recognized by FAP. PCT Publication WO 2013/033396 (incorporated by reference) discloses a FAP-activated prodrug of a proteasome inhibitor, wherein the proteasome inhibitor is linked to a FAP substrate, such that when the proteasome inhibitor is released from the prodrug as a result of cleavage by FAP, the proteasome inhibitor inhibits the proteolytic activity of a proteasome with a Ki of 500 nM or less.